The present invention relates to vector network analyzers. More specifically, the present invention relates to the method and apparatus for efficiently measuring scattering parameters of multiport devices in vector network analysis.
Vector network analyzers (VNAs) are used to determine the transmission and reflection characteristics of various devices under test (DUTs). A transmission or a reflection characteristic is usually referred to as a scattering parameter S. In order to fully characterize a particular DUT, transmission characteristics and reflection characteristics for all combination of two ports are made. In general, for a P port device, this requires P2 scattering parameters, or Sij for each i and j where i goes from 1 to P and j goes from 1 to P. A scattering coefficient is represented as Sij where Sij represents reflection coefficient for port i when i=j and transmission coefficient from port j to port i when ixe2x89xa0j. For convenience, additional subscriptM is used to indicate a measured value, subscriptC for partial error corrected value (corrected for isolation and tracking errors), subscriptN for normalized value, and subscriptA for actual value. For example, SijM indicates measured transmission coefficient from port j to port i.
In order to fully characterize a device, the DUT, having P ports, the VNA performing the tests needs to either (1) have P or more receivers and make P sweeps of the DUT (hardware-intensive approach), or (2) have less than P receivers and make more than P sweeps (time-intensive approach). Here, a sweep may be defined as the process of sending out a source signal (the incident signal) from a given port and measuring detected signals from one or more ports which may include the source port. For example, to characterize a device, the DUT, having three ports (P=3), a VNA having 4 or more receivers, may make the sweeps listed in TABLE 1A. One receiver, Receiver 0, to measure the incident signal, and the other three (Receivers 1, 2, and 3) to measure the signal at each of the three ports. This would be the fastest approach but requires a VNA having the most hardware.
For any VNA, the minimum number of receivers needed is twoxe2x80x94to first to the measure the incident signal and the other to measure the signal at a port. For a two-port VNA measuring a DUT having three ports, the sweeps listed in TABLE 1B are required. This would require the least amount of hardware within the VNA, but would be the slowest approach.
Alternatively, when using a VNA having 3 receivers, the VNA makes the sweeps listed in TABLE 2 to characterize a device having three ports.
For practical implementation of VNAs, there is a trade-off between the hardware-intensive solution (higher cost) and the time-intensive solution (slower process). On the one hand, for the hardware-intensive approach, it is costly to build, operate, and maintain additional hardware required for the large number of receivers. On the other hand, as for the time-intensive approach, it takes longer period of time to fully characterize a device because multiple sweeps are made. Further, the number of required sweeps grows very fast as P, the number of ports of the device, grows. In fact, for a VNA having two receivers, the number of sweeps required to fully characterize a device is P2.
There is a need for a technique and an apparatus to reduce the number of receivers, the number of sweeps, or both to fully characterize a device.
These needs are met by the present invention. According to one embodiment of the present invention, a device under test (DUT) may be characterized by measuring transmission coefficient for transmission of signal from a first port to a second port, SijM. That measurement may be used to determine the actual transmission coefficient of transmission of signal from the second port to the first port, SjiA, without having to measure the transmission coefficient of transmission of signal from the second port to the first port, SjiM. Of course, the measured signal, SijM, may also be used to determine the actual transmission coefficient of transmission of signal from the first port to the second port
According to another aspect of the invention, an apparatus for characterizing a device under test (DUT) is disclosed. The apparatus has a processor and storage connected to the processor. The storage holds instructions for the processor to measure transmission coefficient, SjiM, for transmission of signal from a first port to a second port, and to determine, using that measurement, actual transmission coefficient, SijA, for transmission of signal from the second port to the first port.
Yet another aspect of the invention, a machine readable storage is disclosed. The storage is encoded with instructions causing, when executed by a machine, to measure transmission coefficient, SjiM, for transmission of signal from a first port to a second port; and determine actual transmission coefficient, SijA, for transmission of signal from the second port to the first port.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example, the principles of the present invention.